Harvestmen may look like spiders, but they aren’t. They are arachnids, though. For some reason this came as a shock to me the first time I learned it and I don’t really know why. It’s not like it shook the foundation of my reality. Anyways, the main difference between opiliones and spiders is that spiders have 3 body segments while opiliones only have 2. They also only have only a single pair of eyes.
They also are commonly referred to as Daddy Longlegs, but this can often create confusion because that nickname is also used for the common “cellar spider” Pholcidae.
I’ve often heard the rumor that opiliones are one of the most poisonous arachnids in the world, but are too weak to actually bite you. Apparently this is untrue as they don’t posses fangs, or even venom glands for that matter. They use a fangless chelicerae for grasping and eating food.
One final thought, and I can’t be the only one to think this. I was instantly reminded of the opiliones the first time I came across a “Strider” in the awesome game Half-Life 2.